Helicopters are unique in that their performance, including that of flight, depends on a length of each rotor blade provided on a rotor hub. For example, with heavy lifting, a long rotor blade works best, whereas short rotor blades are desirable for reducing drag and for achieving higher maximum speeds, especially in a compound or coaxial configuration.
In part because of the need for differing performance requirements within a single aircraft at diverse flight conditions, there has long been interest in providing helicopter rotor blades with variable lengths. Altering the diameter of a rotor changes a number of characteristics including blade tip speed, effective lift, landing capabilities, stealth, aircraft speed, reduction in radar profile, fuel efficiency, and more.
There have been attempts to provide helicopter rotors wherein the length of each blade may be adjusted. Most recently, advances in the variable diameter rotor concept have been initiated by Sikorsky Aircraft.
A variable diameter rotor 100 as offered by Sikorsky is illustrated in FIG. 1, by way of example. The rotor 100 has an outward appearance similar to that of a conventional helicopter rotor, but blades 110 thereof telescope in length during flight to increase or decrease rotor disk area. A complex system 120 of differential gears, extension and retraction brakes, and jackscrews are used to extend and retract the rotor blades 110. A majority of the components are provided in a hub (not shown) of the rotor 100, with the jackscrews, for example, housed in the rotor blade 110.
There is recognition in the field of helicopter rotors, that the known system (including differential gears, extension and retraction brakes, jackscrews and the like) used to achieve the desired extension and retraction of helicopter rotor blades is also most subject to failure. Specifically, upon operation of the rotor, the rotor blades encounter huge centrifugal forces, requiring large actuators to retract the rotor blades during flight. The force required to counter the centrifugal force creates friction in the system, and the mechanical components therefore tend to jam because of the extreme friction. Because centrifugal force has previously been believed to be a deterrent to the functioning of variable diameter blades, there has been no recognition to utilize generated centrifugal force as a component of a variable diameter rotor.
Adjustable length blades are known for various other devices. However, these devices cannot be looked to for guidance for various reasons. For example, in a wind turbine, it has been practiced to provide a movable blade section attached to a fixed blade section. In some cases, extended blades are utilized to increase drag in higher wind conditions and in other cases, the extended blades can be used to increase power output. It will be appreciated though, that in all wind turbines, the blade rotation translates wind power into energy, and there is no direct rotation of the turbine blades by an internal power source of the wind turbine. In fact, providing an internal power source to rotate wind turbine blades would be contrary to a function of wind turbines, at the outset. Accordingly, a length of the wind turbine blades is not a function of internal power supplied by the wind turbine, nor rotation of a hub supporting the blades. Instead, wind turbine power is derived from the wind and more specifically rotation of the blades by the wind.
In another device, such as a ceiling fan, it is known to use rotation of a fan hub to pivot a fan blade from a folded position to a fanning position. However, it has not been recognized to linearly extend a length of the fan blade as a result of the fan hub rotation, or to control a length of the fan blade as a result of a rotational speed of the fan hub.
Accordingly, there remains a need for helicopter rotors with variable diameter rotor blades, which will overcome the known deficiencies described.